In many computer network applications, several different types of systems or devices may be connected to a common backplane. From within a computer system enclosure, storage units may connect to the common backplane and from without that enclosure, devices or systems that may desire to connect to the storage units do so through the backplane. The system or device that connects to the storage device through the backplane may do so through an interconnect bus that operates according, for example, the Small Computer Storage Interconnect ("SCSI") protocol, an industry standard. The storage units to which this interconnect bus connects also operate according to the SCSI protocol.
Another protocol which a system or device may use for its interconnection bus is the Digital Small System Interconnect ("DSSI") protocol. The DSSI protocol has been developed by Digital Equipment Corporation, Maynard, Massachusetts, and is the subject of co-pending U.S. application Ser. No. 07/562,433, filed Jul. 19, 1990.
The interconnect bus for using either SCSI or DSSI protocol may be part of an interface. Therefore, the interconnect buses will be referred to as a SCSI bus or a DSSI bus to denote the operable protocol.
Electrical signals that form the SCSI or DSSI bus operate as transmission lines. As such, it is necessary to terminate these transmission lines at both ends with a termination component. This termination component must have an impedance equivalent to the electrical characteristics of the transmission lines.
The DSSI bus has different electrical characteristics than the SCSI bus. For example, the impedance, bus driver, and receiver components are different for each bus. The existence of the different electrical characteristics for the DSSI and SCSI buses results in the DSSI bus having improved signal integrity by preventing voltage overshoots and undershoots. This improves the noise margin between logic "0" and logic "1" values.
With regard to the bus driver components for the two buses, the driver for SCSI bus is capable of sinking a load of up to 48 mA of current, while the driver for the DSSI bus is capable of sinking loads as high as 100 mA of current. Therefore, the DSSI bus drivers can be used to drive a SCSI bus since the DSSI driver components are under stressed when operated to perform this task. However, the DSSI and SCSI buses have distinctive impedances. As such, a storage device operating under the SCSI protocol cannot connect to a DSSI bus and vice versa because the bus termination component could not match the impedance in both directions.
Even though logical signalling of information on a SCSI or DSSI bus is the same, the SCSI bus requires a termination impedance of 132 .OMEGA., while the DSSI bus requires a termination impedance of 85 .OMEGA.. Normally, the electrical characteristics of the termination component associated with either the SCSI or DSSI bus match those of that bus. This, however, causes problems when a system is configured with termination components that are married to either SCSI or DSSI technology because storage units that are supported by storage assemblies may change or at the time of manufacture may not be known. Therefore, every time a storage unit is changed or a decision is reached with regard to storage units, it may result in the necessity to change the termination component to accommodate the particular bus and storage unit connected to it through the termination component.
Changing termination components either at the manufacturing site or in the field requires time and expertise which is not always available. Moreover, the removal or swapping of termination components requires that unused components be saved in case the original storage units are needed again. Mechanical switches to select the proper termination components are cumbersome and expensive.
Accordingly, it would be desirous to have a termination component that is capable of operating at more than one impedance to accommodate more than one bus type to be connected to it.